1. Field of the Invention
This invention relates to a process for producing a polymerized toner, more particularly, relates to a process for producing efficiently a polymerized toner having markedly improved dispersibility of a colorant, homogenous sharp particle diameter distribution and excellent toner properties such as image density and color tone.
2. Description of the Related Art
Electrophotographic process generally forms an electrostatic latent image with electric charge and light exposure on a photoconductive member prepared from a photoconductive material. The formed electrostatic latent image is then developed with a developer (toner) to give a toner image. The obtained toner image is transferred on a recording media such as transfer paper and fixed to give a visible image.
The developers are mainly composed of colored particles (or may be referred as “colored resin particles”) with dispersed a colorant, a charge control agent and a parting agent in a binder resin and roughly classified into one component developer without including carrier particles and two components developer comprising carrier particles. Further, the developers are classified into non-magnetic and magnetic developers whether the colored particles contain magnetic particles or not.
The colored particles are roughly classified into a pulverized toner obtained by pulverization method and a polymerized toner obtained by polymerization method. The pulverization method fuses and kneads the binder resin, the colorant and the other additive components, and then pulverizes and classifies to give the pulverized toner as colored resin powder. The binder resin is synthesized in advance by polymerization of a polymerizable monomer.
While the polymerization method generally polymerizes a polymerizable monomer composition comprising at least the polymerizable monomer and the colorant in an aqueous medium to give the polymerized toner as colored polymer particles. Suspension polymerization method has been mainly used as such a polymerization method, in addition, other dispersion polymerization and emulsion polymerization methods are also known.
Recently, colorization is under rapid progress in electrophotographic procedure and a high quality toner capable to comply with color imaging apparatus is required. The color imaging apparatus is generally equipped with plural image forming parts and each image forming part forms different respective color toner image. Practically, each image forming part successively transfers color toner images of yellow, magenta, cyan and black on the same recording medium. The transferred images are fixed to give a colored image.
As explained above, the color imaging apparatus forms color images by overlapping various color toners and their high transparencies are required. In addition, the color toner is required excellent spectrorefractory feature to improve reproducibility of colors. In addition, the color toner is required to exhibit fixation at low temperature, precise control of positive or negative charge and simple process for production.
The colorant should be pulverized as far as possible and homogenously dispersed in the binder resin to satisfy these requirements. Therefore, sufficient homogenous fine dispersion of the colorants in polymerizable monomer is necessary for production of polymerized toner. Generally, substantially insoluble pigments and dyes have been used for liquid polymerizable monomers. The colorants are conventionally supplied as granules, however, difficult to homogenously disperse in the polymerizable monomers because of insufficient pulverization.
Insufficient dispersion of the colorants in polymerizable monomer composition causes formation of homogenous droplets of the polymerizable monomer composition difficult in aqueous media leading to broad range of particle diameter distribution of the polymerized toner or liable to decline the image density of obtained polymerized toner. Furthermore, insufficient miniaturization and uneven dispersion of the colorant lowers shelf stability of polymerizable monomer composition after dispersion process and liable to cause separation of the colorants during storage.
A number of methods have been proposed for the dispersion of colorants in polymerizable monomer using various media dispersing machines. Practical dispersion process for the dispersion of colorants in monomer system comprising at least a polymerizable monomer by a media type dispersing machine as shown by FIG. 5 is proposed (Japanese Patent Laid-open No. 6-75429).
The media type dispersing machine shown by FIG. 5 is constructed of a cylindrical casing (that is “stator”) 201 having a liquid supplying inlet 202 with equipped plural agitator disks (that is “rotors”) 207 on a driving shaft 210 and contains a number of media particles 208 in the inner space of the casing. A jacket 206 having a cooling medium inlet 204 and its outlet 205 is arranged on the outside wall of the cylindrical casing 201 so as to adjust the inner temperature with the cooling medium.
A mixed solution (slurry) comprising at least a polymerizable monomer and a colorant is continuously introduced in the casing through the liquid supplying inlet 202 by rotation of agitator disks 207 with the rotation of the driving shaft 210 so as to provide strong sharing force on the media particles and the mixed slurry to finely disperse the colorant in the mixed slurry. The mixed solution in which the colorant is finely dispersed (that is “dispersion”) is separated from media particles by a media separating gap separator 209 and transferred to the outside through a liquid discharging outlet 203 on the upper part of casing 201.
The media type dispersing machine shown in FIG. 5 has wide internal space and increased rounding speed of the tip of agitator disks enhances pulverizing capacity and pushes the filled media particles on the inside wall of casing by centrifugal force of the agitator disks. This causes marked packing phenomenon and results in uneven distribution of filled layer of media particles in the apparatus and further causes increased flow of the mixed solution of media particles through thin and less resistant layer. This phenomenon is so called short pass and declines efficacy of pulverization and dispersion, and homogeneity.
In addition, the media type dispersing machine has drawbacks liable to localize the media particles in the media separating gap separator during dispersion treatment and increases inner pressure of the apparatus to lower the efficacy of dispersion.
Heretofore, a media type dispersing machine having a structure shown in FIG. 4 is also known. This media type dispersing machine has a structure composed of a cylindrical casing 101 with a liquid supplying inlet 102 and its outlet 103. In the casing, a driving shaft 110, a rotor 107 on the driving shaft 110, and a media particle separation screen 109 fixed on one side of the cylindrical casing 101 are arranged.
One terminal of the rotor 107 forms a cylindrical structure with plural media particle discharging slits 111 and a media particle separation screen 109 is arranged in the inside of the cylindrical structure. An internal space formed between the inner surface of the casing 101 and the outer surface of rotor 107 contains a number of media particles 108 providing a dispersion chamber. The outside wall of casing 101 is arranged with a jacket 106 equipped with a cooling medium inlet 104 and its outlet 105 so as to control the inner temperature of casing.
The structure of media type dispersing machine shown in FIG. 4 has a narrow space between the inner surface of casing 101 and the outer surface of rotor 107, and slurry supplied through the liquid supplying inlet 102 can be sufficiently dispersed with fully filled media particles 108.
However, this media type dispersing machine has a fixed media particle separation screen 109 and transportation of dispersed slurry to the outside through the media particle separation screen 109 and the liquid outlet 103 causes massive localization of media particles on the media particle separation screen 109 and declines the efficacy of dispersion.
A media particle stirring type wet dispersing machine including plural spherical media particles in the chamber is proposed as a dispersing procedure of fine particles of colorant in the liquid polymerizable monomer composition (Japanese Patent Laid-open 2001-166531).
This media particle stirring type wet dispersing machine has a structure including media particles between a rotating rotor and a separator. A liquid monomer mixture comprising fine particles of colorant is introduced through a liquid supplying inlet to the center of the inner chamber, disperses the colorant in the mixture by centrifugal force generated by the rotation of the rotor and the media particles and the resultant mixture is discharged outside from the inner chamber through the slits of separator.
However, no sufficient dispersion of the colorant can be obtained even by the use of the media particle stirring type wet dispersing machine and the media particles localize on the slits of separator, elevates the inner pressure and the dispersion efficacy is liable to decline.
A method using a high pressure percussion type dispersing machine ULTIMAIZER®. (HJP3000, Sugino Machine Ltd.) is proposed for the other method to disperse the colorants in polymerizable monomers (Japanese Patent Laid-open 1998-301333). The ULTIMAIZER® dispersing machine does not use media particles, loads pressure energy on the liquid, divides the stream in two pathways and forces countercurrent collision at the joint of two streams to pulverize the subject.
Application of ULTIMAIZER® dispersing machine in countercurrent collision causes marked temperature elevation of polymerizable monomer mixed solution comprising the colorant and may start polymerization during dispersion treatment. Thus, no sufficient pressurization in countercurrent collision can be performed.